1. Field of the Invention
The present invention relates to a method of producing catalytic materials and to catalytic materials produced by this method.
2. Description of the Prior Art
Among the most enduring paradigms of catalysis are the methods of supported metal catalyst production. For decades most of these materials have been made via variations on the time consuming and `dirty` incipient wetness technique. High surface area materials are saturated with solvent (generally water) containing dissolved metal salts. The solvent is then evaporated and the salt decomposed by heating. In general the catalyst is then reduced. A relatively small number of supported metal catalysts are made by other classic techniques, in particular precipitation and ion exchange. Occasionally alternative approaches are explored. For example, following the lead of Parkyns (1) and others (2), a great deal of effort has gone into generating metal particles by partially/fully decomposing organometallic clusters. The approach has proven to be expensive and largely futile. The particles produced in that manner are generally found to be structurally and catalytically similar to those produced using the far less expensive incipient wetness method (3).
In the last two decades materials processing using plasmas has dramatically increased. A variety of plasma processing techniques are now employed in the production of virtually all integrated circuits. Plasmas are also used to improve `materials` processing technology, for example, in the production of diamond films (4,5), as an alternative to flame techniques for the production of high quality titanium dioxide, and even to create polymer films with unique characteristics (6,7). Finally, plasma techniques have been employed with modest success to create truly novel materials, such as carbo-nitride films (8,9).
Yet, there is only a single prior example of the use of plasmas to create novel `supported` catalytic materials (10) and this example is clearly not a `model` process for general supported catalyst production. There are also a few examples of catalytic processes accelerated by plasmas (11), presumably via the (homogeneous) generation of radical species (12), as well as examples of plasmas `activating` catalysts by removal of poisons or accelerating reduction (13-15). There are also examples of the use of plasmas to create thin support films (model catalysts) which are later metal impregnated using conventional `wet` chemistry (16), and `opposite examples` that is, systems in which plasmas spray metals onto conventionally prepared oxide films (17).
Only the single use of a plasma to create supported metal catalysts is of direct bearing on the present invention. The method employed in earlier work was significantly different from that proposed herein. Specifically, in the earlier work, catalysts were created by D-C discharge across a flowing stream of hydrocarbons. The discharge `carbonized` some of the hydrocarbons and resulted in volatilization of metal (nickel) from the electrodes.
Particle production/treatment techniques in atmospheric plasmas can be broken into three categories: i) particle `treatments` which do not involve a change in the particle chemistry, ii) particle production in which the final particles do not incorporate any of the gases used to `fire` the plasma and iii) particle production in which the plasma gas phase is incorporated in the final structure. Film formation using plasmas operating at `low` pressures (&lt;100 Torr) are not relevant to the present invention. The focus of this invention is particle rather than film fabrication.
Reports on particle `treatments` (category (i) above) are the most common. Most reports involve the use of commercial plasma torches, both DC arc and radio frequency, running on flowing inert gases (generally argon) to which metal particles are fed. The metal particles are often used to create high density films to coat other materials, often as some type of protective barrier. Some commercial processes in this category are thirty years old (18). Generally there is no attempt in this technology to modify the structure of the particles. The driving concept is simply to use the high energy density of plasmas to `melt` the particles such that the metal can adopt the form of the target surface upon impact/quenching (19,20). There have been instances in which the technology has been employed to create structures somewhat different from the original feed. For example, it was recently demonstrated that if two torches, each with a different material, are run to spray the same surface simultaneously, the result is a form of `laminated` film (7,21). In fact, there are several examples of the use of particle fed torches to create films with an intimate `atomic scale` mixing of the two materials (19,22,23).
The most relevant work is metal evaporation in plasmas. Repeatedly it has been found that metal particles are completely atomized in atmospheric pressure torches. One group injected pure iron and aluminum powders in the micron size range into a high power (32 kW) thermal arc plasma at the rate of about 5 gms/min. In the plasma it was presumed that the original particles completely evaporated and new particles on the order of 100 nm in size nucleated and grew in the afterglow (24). Other groups have also recently shown that micron sized iron particles are atomized during passage through a torch (25). The particles which are captured and examined are presumed to form by nucleation and growth of atomic species in the afterglow region.
There are a variety of methods that have been employed to make category II particles (26). One method is to sputter a target metal with a flowing, but chemically inert plasma (e.g. Argon). The particles are then collected downstream using filters, etc. Another method is to make solid, well mixed, beds consisting of two materials. These mixtures are converted to alloys, in particulate form, in gas (no flow) thermal plasma systems. In particular there have been a number of reports on the generation of carbides in this manner (27). Yet another example of the use of plasmas to make particles involves injecting molecular species into a plasma. In the hot zone of the plasma the original molecule is destroyed, and particles probably form in the afterglow during cooling. In our own laboratory, we created iron nanoparticles by injecting an aerosol stream containing liquefied ferrocene into a low pressure microwave generated (argon or hydrogen) plasma (28).
The greatest number of reports in which particles are created by some complex chemistry in the plasma zone (category III) involve the creation of carbides and nitrides. Research in this area is driven by the perception that carbide or nitride production using plasma technology has solid commercial potential.
In our own laboratory (unpublished), we have succeeded in creating aluminum nitride particles by injecting 1 micron aluminum particles through the center of a nitrogen plasma generated using our torch. Other groups did similar work at a much earlier date. Indeed, Vissokov and Brakalov did nearly identical work (29). They postulated that the original aluminum particles were completely atomized, and that AlN particles formed during rapid nucleation and growth in the rapid cooling region of the afterglow. This analysis is consistent with the findings (both theirs and ours) that the final AlN particles are orders of magnitude smaller, on a volume basis, than the input aluminum particles. Other workers made AlN from aluminum particles and ammonia (30,31).
There are numerous examples of methods to create carbides (15,27,32,33) and plasma methods are said to be both significantly faster and more energy efficient than alternative fabrication techniques. One of the more relevant methods involves the injection of particles into a plasma torch operating at atmospheric pressure which contains hydrocarbon molecules. According to the inventors of this technology (34) the metal particles (e.g., Ti, Mg, Si) completely atomize in the hot plasma region and then in the cooling afterglow, nucleate new particles which incorporate carbon atoms created during the decomposition of the hydrocarbon molecules.
In view of the above review of prior art, there is no evidence in prior literature of the use of plasma torches to create traditional supported metal catalysts, although there is a considerable history of the use of plasma torches to atomize metal particles.
Further, there is considerable interest in the use of plasma treatments to sinter micron scale oxide particles together to form high density solids. The impetus for this interest was the finding by Bennett and co-workers (35) that in plasmas, alumina compacts more rapidly and at significantly lower temperatures than it compacts when treated thermally. Since that time several groups have confirmed that plasma processing accelerates the sintering of alumina and other oxides (36-38).